Birth order, sibship size, and risk of atopic dermatitis, food allergy, and atopy: A systematic review and meta‐analysis

Abstract Background Atopic dermatitis and food allergy are two frequently concomitant manifestations of the presence of atopy. A substantial number of studies have been published on the association of birth order and sibship size (number of siblings) with atopic dermatitis, food allergy, and atopy. The present work is the first systematic synthesis of the existing literature on this topic. Methods Fifteen databases were searched. Screening, data extraction, and quality assessment were performed by independent pairs. Comparable numerical data were statistically synthesized using random‐effects robust variance estimation. Results In total, 114 studies were included out of 8819 papers obtained from database searches. Birth order ≥2 versus 1 was associated with lower risk of ever atopic dermatitis (pooled risk ratio [RR] 0.91, 95% CI 0.84–0.98), current food allergy (RR 0.77, 95% CI 0.66–0.90), and positive skin prick test (SPT) to common aeroallergens (RR 0.86, 95% CI 0.77–0.97). Sibship size ≥2 versus 1 was associated with decreased risk of current atopic dermatitis (RR 0.90, 95% CI 0.83–0.98), ever atopic dermatitis (RR 0.92, 95% CI 0.86–0.97), and positive SPT to common aeroallergens (RR 0.88, 95% CI 0.83–0.92). No putative associations were seen regarding atopy assessed through allergen‐specific immunoglobulin E with common allergens. Conclusion The presence of siblings and being second‐born or later may decrease the lifetime risk of atopic dermatitis and food allergy, albeit marginally. Similar association was seen with SPT sensitization. However, significant protection was not found for IgE sensitization.


| INTRODUCTION
Atopic dermatitis and food allergy are two common atopic diseases. 1 Atopic dermatitis, the most common chronic relapsing inflammatory skin disease, is particularly often seen in children, with reports indicating that up to a third are affected in Northern Europe. Studies in adults estimate that it affects, on average, close to 10% of the general population in the Europe and United States. [2][3][4] An increase in atopic dermatitis has been noted in several regions of the world in recent decades, including Africa, eastern Asia, and parts of Europe. 5 Similarly, the most widely recognized form of food allergy, 6 immunoglobulin E (IgE)-mediated food allergy, affects 1%-10% in the general population, with higher prevalence seen in Western than non-Western countries and in young children. [7][8][9][10] Also food allergy has increased in prevalence in recent decades, particularly in developed countries, 9 but a substantial heterogeneity remains in distribution and recent trends between continents and countries. 7,11,12 Atopic dermatitis and food allergy are associated 13 and commonly co-exist in the same individual, 14 not seldom as components in the "atopic march," which typically begins with atopic dermatitis, subsequently progressing into food allergy and other atopic diseases. [15][16][17] Both conditions, especially in early life, are common manifestations of atopy, which is an immunological predisposition characterized by exaggerated IgE production against otherwise commonly innocuous environmental allergenic molecules, 18 but such sensitization is asymptomatic in some individuals, meaning that no symptoms are seen upon exposure to the allergen. 19,20 The underlying reasons for the heterogeneous triggers, clinical presentations, and trajectories of atopic dermatitis, food allergy, and atopy, are not yet fully elucidated, but are thought to be constituted of a complex set of interrelated (epi)genetic, immunological, and environmental factors. [21][22][23] As a substantial increase in the prevalence of atopic diseases has been reported in developed as well as rapidly developing and industrializing/urbanizing countries, 8,24 the role of changes in lifestyle and environment in these regions as contributors to the observed increase have been of interest in research. Strachan popularized the "hygiene hypothesis," which suggested that the presence of older siblings at home may confer protection against the development of allergy. 25 While initially highlighting the association with allergic rhinitis, a substantial body of research has subsequently been published on the role of sibship composition with related outcomes. We undertook the present systematic review and meta-analysis to synthesize the existing literature on the association between birth order and sibship size (number of siblings) and risk of atopic dermatitis, food allergy, and atopy. To the best of our knowledge, this is the first systematic review of this topic.

| METHODS
This work was performed following a prospectively registered (International prospective register of systematic reviews [PROSPERO]; CRD42020207905) and published 26 protocol, based on the Preferred Reporting Items for Systematic Review and Meta-Analysis Protocols (PRISMA-P) guidelines. 27 Reporting of the present work was based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 28 checklist (Supporting Information S1: Table E1) and the Meta-analysis of Observational Studies in Epidemiology (MOOSE) 29 reporting guidelines (Supporting Information S1: Table E2).

| Inclusion and exclusion criteria
Studies-regardless of sample size or medical/sociodemographic background of subjects-fulfilling the following criteria were eligible for inclusion:

| Data sources and search strategy
We searched the following databases without restriction on publica- Non-English articles were translated with Google Translate. 34 The database searches were complemented by hand-searching of reference lists in the included studies. Queries used to perform the searches are presented in Supporting Information S1: Table E3A-I.

| Study selection and data extraction
EndNote X9 (Clarivate Analytics, 2020) was used to host-retrieved records and perform de-duplication, following a semi-automated method proposed by Bramer et al. 35 Pairs of reviewers independently screened records following a two-step approach, based on (1) title and abstract, and (2) full-text of potentially relevant articles.
Similarly, pairs of reviewers independently extracted data from the included studies. The following data were extracted from each article: main author, year of publication, study design, the source of subjects (e.g., among male conscripts undergoing medical examination at recruitment offices or from the general pediatric population), country, number, and age of subjects, definition/assessment method of exposure and outcome, and point estimates with 95% confidence interval (95% CI) for relevant exposure-outcome pairs. After each step during screening and data extraction, the decisions were unblinded and compared for agreement. A third reviewer (BIN) arbitrated when needed.

| Quality assessment
Risk of bias in the included studies was assessed using the Effective Public Health Practice Project 36 tool, with slight modifications used in a work by Smith et al. 37 to better fit the investigated data. Based on the rating of "weak," "moderate," or "strong" in six domains (study design, selection bias, confounding, blinding, data collection, and withdrawals/dropouts), an overall rating was given based on the number of "weak" domains: "weak" if more than one, "moderate" if one, and "strong" if none of the domains had given a "weak" rating.
The risk of bias assessment was performed independently in pairs of reviewers. After completion, differences were discussed, and a third reviewer (BIN) arbitrated when needed.

| Data synthesis and statistical analysis
Relevant data from the included studies are summarized and presented in tables of characteristics for each outcome (atopic dermatitis, food allergy, and atopy). The overall findings were synthesized narratively. Comparable numerical data-based on similarity in exposure, outcome, assessment methods and subject characteristics-were also synthesized statistically using meta-analysis with random-effects robust variance estimation (RVE). 38 RVE enables the inclusion of data with various structures of dependency in the same meta-analysis model, for example, different sibship sizes or birth orders compared to the same reference group (single children and first-born, respectively), which constituted the majority of dependent estimates in our data.
The meta-analyses were performed using the correlated effects model, with small sample-correction (to increase accuracy) 39  ≥2 studies. 40 For birth order, being first-born constituted the reference group, and for sibship size the reference group was single children. The reciprocal of the point estimate as well as the lower and upper bounds of the 95% CI was calculated in cases where the reference group was of higher cardinality, for example, birth order <3 versus ≥3. Forest plots from the meta-analyses were produced using the forestploter R package. 41 Subgroup analyses were performed where comparable data were available from ≥4 studies in ≥2 subgroups, 42 to investigate sources of heterogeneity in findings, based on (a) study design; (b) exposure cardinality (e.g., sibship size 4); (c) overall rating; (d) year(s) of data collection, divided into <2000 and ≥2000; (e) World Bank classification 43 of the study country in the year of publication into "high income," "upper-middle income," "lower-middle income," and "low income" economy; (f) subject age, divided into children (<18 years) and adults. Additionally, sensitivity analyses were performed to assess the robustness of our pooled estimates by re-running the meta-analysis only on studies with (a) a "moderate" or a "high" overall rating and (b) physician/clinical outcome assessment (or report thereof). Sensitivity analysis was also performed based on the rho value, by re-running the meta-analysis using rho values ranging from 0 to 1, with 0.2 increments, using the sensitivity() function from the robumeta R package. 39 Finally, publication bias was assessed in exposure-outcome pairs with 10 studies 44 using the metafor R package 45 and a two-fold approach: (1) visual assessment of funnel plots for signs of asymmetry; (2) statistical assessment using Egger's regression test 46 and Begg and Mazumdar rank correlation, considering corresponding p-values <0.05 as statistically significant.
Risk ratio (RR) was used as the measure of effect due to ease of interpretation. [47][48][49] Prevalence ratio data were used without conversion as these are calculated identically with RR. 47 Odds ratio and hazard ratio data were converted to estimates of RR in studies where the outcome prevalence was ≥15% (at the end of follow-up) using the following formulae: Inter-study heterogeneity was assessed with the I-squared (I 2 ) statistic, 50 while inter-study variance was estimated with Tausquared (τ 2 ). 51 Meta-analysis outputs with Satterwhite degrees of freedom (df) <4 were considered unreliable. 39 All R scripts and data used to perform the meta-analyses are freely available at the Open Science Framework (https://osf.io/fp3rw/).

| RESULTS
The database searches yielded 17,466 records. Of these, 114 reports based on 102 studies met the full inclusion criteria (Figure 1).

| Study characteristics
Across the included studies, data from 75 countries and a total of >2 million subjects were available ( Figure 2). The most common LISIK ET AL. study design was cross-sectional (n = 55), followed by cohort studies (n = 43), case-control studies (n = 11), nested case-control studies (n = 4), and one case-cohort study. Most studies received a "moderate" (n = 51; 45%) or "strong" (n = 47; 41%) overall rating, while 16 (14%) were rated "weak" ( Figure 3A, Supporting Information S1: Table E4). The vast majority of studies were published after the turn of the millennium. Similarly, the assessed quality increased substantially in studies published in recent years ( Figure 3B,C). See Supporting Information S1: Table E5A-C for detailed characteristics of the included studies.

| Ever atopic dermatitis
Ever atopic dermatitis was assessed with meta-analysis in 16 studies for birth order and seven studies for sibship size ( Figure 5, Supporting Information S1: Figure E2A,B). The overall pooled effect size indicated a 9% lower risk with birth order ≥2 versus 1 (RR 0.91, 95% CI 0.84-0.98) and an 8% lower risk with sibship size ≥2 versus 1 (RR 0.92, 95% CI 0.86-0.97). Point estimates decreased relatively consistently with increased birth order and sibship size, respectively.

| Current food allergy
The current food allergy was assessed with meta-analysis in six studies for birth order (Figure 6, Supporting Information S1: Figure   E3). The pooled effect size indicated that having at least one older sibling was associated with a 23% lower risk of the outcome (RR 0.77,

| Ever food allergy
There were too few comparable studies to perform meta-analysis on this outcome or to make a clear assessment narratively, as the included studies varied substantially in exposures investigated and subject characteristics (Supporting Information S1: Table E5B). In a study on cow's milk allergy specifically, a reduced risk of the outcome was seen in subjects with ≥4 older siblings. 52 On the other hand, a register-based study on any food allergy diagnosis found no association with birth order ≥2 versus 1. 53

| Atopy
Atopy as defined through sIgE levels above the traditional 56,57 threshold of 0.35 kU A /L was assessed in seven studies for birth order (using combinations of aeroallergen, some of which included common foods) and five studies for sibship size (using combinations of aeroallergens; Figure 7, Supporting Information S1: Figure   E4A,B). The effects of both birth order ≥2 versus 1 and sibship size ≥2 versus 1 were comparable (RR 0.89, 95% CI 0.79-1.01 and RR 0.92, 95% CI 0.79-1.08, respectively), each statistically nonsignificant. In subgroup analysis, a trend with decreasing point estimate with increased cardinality of the exposure could be discerned. Heterogeneity was moderate for both birth order (I 2 = 51.9%, τ 2 = 0.01) and sibship size (I 2 = 67.0%, τ 2 = 0.02). For atopy as defined through positive SPT, the pooled effect size from the 12 studies on birth order (using combinations of aeroallergens, some of which included common foods; Figure 8A, Supporting Information S1: Figure E5A) indicated that birth order ≥2 versus 1 was associated with 14% lower risk of the outcome (RR 0.86, 95% CI 0.77-0.97). In the studies including food allergens, the association was not statistically significant, however. Similarly, the pooled effect size of the eight studies on sibship size (using combinations of aeroallergens; Figure 8B, Supporting Information S1: Figure E5B) indicated that sibship size ≥2 versus 1 was associated with 12% F I G U R E 4 Forest plot for birth order ≥2 versus 1 (A) and sibship size ≥2 versus 1 (B) in relation to current (in last year) atopic dermatitis. df, Satterwhite degrees of freedom; I 2 , I-squared; N, number of subjects (if not available, the number of subjects for the most similar exposureoutcome pair or for the whole study is stated); RR (95% CI), risk ratio (95% confidence interval); τ 2 , Tau-squared. lower risk of the outcome (RR 0.88, 95% CI 0.83-0.92). Heterogeneity was moderate for birth order (I 2 = 67.1%, τ 2 = 0.02) and low for sibship size (I 2 = 39.1%, τ 2 = 0.01).

| Publication bias and sensitivity analysis
The p-value of 0.04 from the Egger's regression test for atopic dermatitis by birth order indicated asymmetry (Supporting Information S1: Figure E6, Table E6). Seven estimates were filled on the right side with the trim-and-fill method (Supporting Information S1: Figure   E7), but following visual inspection of the corresponding funnel plots, it is unlikely, given the overall concentrated distribution of published and filled results around the top center, that the pooled estimates are biased, although slight publication bias cannot be ruled out. Two estimates were also filled in for allergic sensitization (SPT) by birth order (Supporting Information S1: Figure E7), but the non-significant p-values from the statistical tests (Supporting Information S1: Table   E6) and the weak asymmetry similarly suggest that the results are most likely not biased.
Sensitivity analyses by excluding studies with overall "weak" rating and those with self-reported outcome assessment did not produce substantially different pooled estimates. For the sensitivity analysis by physician/clinical outcome assessment on current atopic dermatitis by sibship size, the 95% CI became wider, no longer indicating a statistically significant association. However, the F I G U R E 5 Forest plot for birth order ≥2 versus 1 (A) and sibship size ≥2 versus 1 (B) in relation to ever atopic dermatitis. df, Satterwhite degrees of freedom; I 2 , I-squared; N, number of subjects (if not available, the number of subjects for the most similar exposure-outcome pair or for the whole study is stated); RR (95% CI), risk ratio (95% confidence interval); τ 2 , Tau-squared.

F I G U R E 6
Forest plot for birth order ≥2 versus 1 in relation to any current (in last year) food allergy. df, Satterwhite degrees of freedom; I 2 , I-squared; N, number of subjects (if not available, the number of subjects for the most similar exposure-outcome pair or for the whole study is stated); RR (95% CI), risk ratio (95% confidence interval); τ 2 , Tau-squared. LISIK ET AL. point estimate was lower in this study subsample, which only consisted of four studies, thus likely being the result of low statistical power (Supporting Information S1: Table E7). Similarly, a marginally wider 95% CI, most likely due to the same reason, was seen for all atopic dermatitis by birth order, excluding "weak"rated studies (Supporting Information S1: Table E7). Sensitivity analyses by rho did not notably affect the effect sizes (Supporting Information S1: Table E8).

| Summary of key findings
The present work constitutes a comprehensive synthesis of the global literature on this topic. The available data indicate that having (older) siblings was only marginally associated with lifetime risk of having atopic dermatitis, with a weaker impact on current atopic dermatitis. In contrast, the presence of older siblings was associated with a substantially decreased risk of current food allergy, albeit with only a few studies. In terms of allergic sensitization (older), siblings appeared to be marginally associated with protection against atopy to common aeroallergens assessed with SPT, but the association was

| Comparison of findings to previous studies
To the best of our knowledge, the present work is the first systematic review and meta-analysis synthesizing the literature on the association between birth order and sibship size with risk of atopic dermatitis, food allergy, and atopy.

F I G U R E 7
Forest plot for birth order ≥2 versus 1 (A) and sibship size ≥2 versus 1 (B) in relation to allergic sensitization assessed by measurement of allergen-specific immunoglobulin E (sIgE) levels to palettes of common food allergens and aeroallergens (A) and common aeroallegergens (B). df, Satterwhite degrees of freedom; I 2 , I-squared; N, number of subjects (if not available, the number of subjects for the most similar exposure-outcome pair or for the whole study is stated); RR (95% CI), risk ratio (95% confidence interval); τ 2 , Tau-squared.

| Interpretation of findings
Both the presence of siblings and being second-born or later was associated with a marginally lower risk of atopic dermatitis. A similar strength in the association was seen between the presence of siblings and current atopic dermatitis, but the association was non-significant in second-born or later. As atopic dermatitis is more commonly seen in childhood (particularly early childhood) with the majority no longer experiencing symptoms in adulthood, 3,19 and that the strongest effect was seen in children in regard to current atopic dermatitis, it may be that the effect of the association is limited to certain phenotypes of the disease 62-64 or a specific time-window.
While the protective effect of birth order above one on the current food allergy was clearer, it was also based on a relatively small set of studies. Thus, while a 23% reduction in the risk of current food allergy was seen, it is difficult to assess the robustness and generalizability of this association.
The association between sibship composition and atopy was relatively similar in terms of exposure, but the association was weaker for both exposure types in relation to sIgE compared with SPT outcomes. Age may be part of the explanation in this case, 65 similar to in the studies on atopic dermatitis, as substantially more studies using sIgE were on adults, in contrast to studies using SPT, where a clear majority were children. Furthermore, while both birth order ≥2 versus 1 and sibship size ≥2 versus 1 indicate the same, albeit weak and possibly practically insignificant, effect, the slight difference in strength and precision of the association may be due to the differences in sensitivity and specificity of the (arbitrary) cut-offs in the different assessment methods. 57 All in all, the findings indicate that for the investigated outcomes, the association with sibship composition is weak, if practically meaningful at all. The causes for this may be, for example, the heterogeneity of allergic sensitization and these atopic diseases or changes in environmental factors that we were unable to account for the meta-analyses.

| Clinical and research implications
Although a protective effect of the presence of (older) siblings was seen across all outcomes, the strength of the association varied F I G U R E 8 Forest plot for birth order ≥2 versus 1 (A) and sibship size ≥2 versus 1 (B) in relation to allergic sensitization assessed by skin prick tests (SPT) to common food allergens and aeroallergens (A) and common aeroallergens (B). df, Satterwhite degrees of freedom; I 2 , Isquared; N, number of subjects (if not available, the number of subjects for the most similar exposure-outcome pair or for the whole study is stated); RR (95% CI), risk ratio (95% confidence interval); τ 2 , Tau-squared. LISIK ET AL.
-9 of 12 substantially between outcomes and age groups. Thus, while our findings partly support the "hygiene hypothesis"-namely that early life cross-infection between siblings can modulate the immune systems in such a way that the risk of allergy development is reduced 66 -they particularly highlight the complex underlying pathophysiological mechanisms and heterogeneous clinical presentations and trajectories of allergic sensitization and atopic diseases alike. 62,67 Our synthesis can potentially be used as a stepping-stone in furthering our understanding of the underlying mechanisms driving allergic sensitization and allergy development, as well as to direct future epidemiological research in terms of environmental factors in relation to the investigated diseases.

| CONCLUSION
Our findings indicate that having siblings and being second-born or later is associated with a marginal reduced risk of lifetime risk of atopic dermatitis. Likewise, a higher birth order is associated with roughly 20% lower risk of current food allergy. Allergic sensitization defined by SPT to common aeroallergens is marginally rarer in those with siblings or second-born or later, while the association for sensitization measured using sIgE did not reach statistical significance. Atopic diseases are heterogeneous and multifactorial, so it is likely that sibship composition only plays a marginal role in the risk of these diseases.

DATA AVAILABILITY STATEMENT
Code and data needed to reproduce our findings are available freely at https://osf.io/fp3rw/.